As addressed in JP05-178231A (hereinafter “Patent Document 1”), in a rear wheel steering system that steers the left and right rear wheels in a symmetric manner by laterally displaying the left and right suspension arms, to steer the rear wheels into a toe-in state depending on the dynamic condition of the vehicle such as the vehicle speed, front wheel steering angle and braking state, and change the toe-in angle in a progressive manner depending on the vehicle speed and front wheel steering angle. According to this technology, the turning capability of the vehicle may be improved in low speed range, and the stability of the vehicle can be improved in high speed range. Also, the tail swaying tendency of the vehicle during a turn and the reduction in the stability of the vehicle under a brake can be effectively prevented.
More recently, with the aim of improving the turning capability and the stability of the vehicle, various four wheel steering vehicles incorporated with a rear wheel toe angle varying device that allows the rear wheel toe angle to be individually changed have been developed. The rear wheel toe angle varying device may be constructed in a number of different ways. An electric actuator may be interposed between the vehicle body and a lateral link or a trailing link of the suspension system supporting each rear wheel as disclosed in JP09-30438A (hereinafter “Patent Document 2”). In a double wishbone suspension system, a knuckle which is supported by the vehicle body via an upper arm and a lower arm using a ball joint in each of the arms may be additionally connected to the vehicle body via an electric actuator as disclosed in JP2008-164017A (hereinafter “Patent Document 3”). In any of such rear wheel toe angle varying devices, the toe angle of each rear wheel can be variably and individually controlled.
A typical electric actuator for a rear wheel toe angle varying device consists of a linear actuator using an electric motor and a feed screw mechanism. The present applicant previously proposed an invention for preventing the inadvertent detachment of an output rod without increasing the size of the linear actuator (patent document 3). When a vehicle component is driven by such an electric actuator, a static friction is caused between the male thread and female thread of the feed screw mechanism, and this static friction impairs the tracking performance of the electric actuator when the actuator is operated from a stand still condition. To overcome the static resistance and improve the tracking performance of the linear actuator, it may be possible to increase the duty ratio of a PWM controlled electric actuator when starting the action of the actuator. Also, JP2006-299810A (hereinafter “Patent Document 4”) describes a drive motor for driving a cooling fan.